There are various types of color displays that have been in practical use. Thin displays are classified broadly into (i) self-luminous displays such as PDPs (plasma display panels) and (ii) nonluminous displays typified by LCDs (liquid crystal displays). A known example of an LCD, i.e., a nonluminous display is a transmissive LCD having a backlight disposed on the back surface side of a liquid crystal panel.
FIG. 14 is a cross-sectional view illustrating a typical structure of a transmissive LCD. This transmissive LCD has a backlight 110 disposed on the back surface side of a liquid crystal panel 100. The liquid crystal panel 100 includes: a pair of transparent substrates 101 and 102; a liquid crystal layer 103 disposed between the transparent substrates 101 and 102; and polarizing plates 104 and 105 provided on the respective outer surfaces of the transparent substrates 101 and 102. The liquid crystal panel 100 further includes a color filter 106 so as to carry out a color display.
Although not illustrated in FIG. 14, each of the transparent substrates 101 and 102 has an electrode layer and an alignment film which are on its inner surface. In response to controlling of a voltage that is applied to the liquid crystal layer 103, the amount of light that travels through the liquid crystal panel 100 is controlled in individual pixels. That is, the transmissive LCD carries out a display control by controlling the amount of light, emitted from the backlight 110, that travels through the liquid crystal panel 110.
A white backlight, containing the wavelengths of three colors RGB necessary for a color display, is mainly used as the backlight 110. The backlight 110 is combined with the color filter 106 so as to adjust the transmittance of light that transmits each of the colors RGB. This makes it possible to arbitrarily set the luminance and hue of a pixel. It should be noted that some backlight 110 includes light sources for the respective colors RGB.
For example, according to the above LCD, the liquid crystal panel 110, including pixels corresponding to RGB regions of the color filter 106, has a shutter function. The shutter function controls the transmittance of light that travels through each of the pixels in accordance with the display information to be outputted. Specifically, the shutter function controls the transmittance of the light that travels through each of the pixels by controlling the transmittance by a predetermined step which falls within the range of 0 to 100%. This causes the intensity of the light that travels through each of the pixels to be controlled. Theoretically, in cases where 100% of the light emitted from the backlight 110 is transmitted, the intensity of a color component corresponding to the light emitted from the backlight is outputted from a corresponding pixel as it is, thereby obtaining a maximum luminance. On the other hand, a transmittance of 0% causes a black display. According to the ordinary transmissive LCD that is configured so that the liquid crystal panel 110 has the shutter function causing a display control, the backlight 110 continues to emit light at a certain level of luminance.
In such a backlight, fluorescent tubes (such as cold-cathode tubes) are mainly employed as a light source thereof. A fluorescent tube is structured to have electrodes at both inner ends thereof, and as such, needs to be made higher in discharge lighting voltage, especially when it is applied to a large-sized liquid crystal panel. Such a high discharge lighting voltage is achieved by a structure in which a high voltage boosted by the output transformer of an inverter is applied to an electrode of the fluorescent tube via a capacitor.
There are no problems as long as a fluorescent tube performs a lighting operation properly. However, once the fluorescent tube has failed in such a structure that a high voltage is applied to the fluorescent tube, there occur various problems in addition to stoppage of lighting. Examples of a failure in the fluorescent tube include the life time of the fluorescent tube, breakage of the fluorescent tube, a bad electrical contact of a connector via which the inverter and the electrode of the fluorescent tube are connected with each other, and disconnection of a lead wire. Upon occurrence of such a failure in the fluorescent tube, the output of the transformer is put in an unloaded condition, whereby the output voltage of the transformer is abnormally increased, posing various dangers such as discharge, firing, and an electric shock to an operator.
Proposed in view of this is a technique for preventing an abnormal increase in output voltage in the case of a failure in a fluorescent tube. Patent Literature 1 discloses an example of such a technique. Specifically, a tube current detection circuit that detects, in accordance with a tube current flowing through a fluorescent tube, whether the fluorescent tube is lighting or not lighting is provided, and in the absence of a tube current, a protection circuit that shuts off supply of power to an inverter circuit and forcibly stops output of the inverter circuit is activated. This makes it possible to prevent an abnormal increase in transformer output voltage.